Adjusting survey points post-casing for improved wear estimation

Abstract

Disclosed embodiments include a method for estimating casing wear including the operations of: obtaining locations of survey points along a borehole, said survey point locations representing a borehole trajectory; casing at least a portion of the borehole with a casing string; deriving locations of adjusted survey points that represent a casing trajectory along said portion of the borehole, the casing trajectory being different from the borehole trajectory; estimating, as a function of position along said casing string, a side force of a drill string against the casing string; computing, as a function of position along the casing string, casing wear based at least in part on the side force; and generating a notification of any positions where casing wear exceeds a threshold.

What is claimed is: 
     
       1. A method for estimating casing wear, comprising:
 obtaining locations of survey points along an open portion of a borehole, said survey point locations representing a borehole trajectory of the borehole; 
 casing at least a portion of the borehole with a casing string to form a cased portion of the borehole; 
 deriving, using the survey point locations, locations of adjusted survey points along one or more portions of the cased portion of the borehole using a model that accounts for a bending stiffness of the casing string; 
 fitting a casing trajectory to the locations of the adjusted survey points, the casing trajectory representing a path of the casing string along the one or more portions of the cased portion of the borehole, the path of the casing trajectory different from a path of the borehole trajectory; 
 estimating, using the casing trajectory fitted to the locations of the adjusted survey points, a side force of a drill string against the casing string as a function of position along said casing string; 
 computing, as a function of position along the casing string, casing wear based at least in part on the side force; and 
 generating a notification of any positions where casing wear exceeds a threshold. 
 
     
     
       2. The method of  claim 1 , wherein said model is a stiff string model or a finite element model. 
     
     
       3. The method of  claim 1 , wherein said deriving further accounts for one or more casing string centralizers on the casing string. 
     
     
       4. The method of  claim 1 , wherein said deriving is based at least in part on a pre-casing caliper log. 
     
     
       5. The method of  claim 1 , wherein said deriving is based at least in part on a post-casing survey log. 
     
     
       6. The method of  claim 1 , wherein said estimating accounts for a bending stiffness of the drill string. 
     
     
       7. The method of  claim 1 , wherein said estimating employs a soft-string model. 
     
     
       8. The method of  claim 1 , wherein said computing is further based on at least one of a wear factor, a drill string property, a rotational speed of the drill string, and a drilling time. 
     
     
       9. The method of  claim 8 , further comprising: upon determining that the casing wear exceeds a threshold, providing a recommendation that includes modifying or limiting at least one of the side force, the wear factor, the rotational speed, and the drilling time, to a value that would reduce the computed casing wear below the threshold. 
     
     
       10. The method of  claim 1 , wherein the casing wear is computed as a casing string wear volume or a casing string wear groove depth. 
     
     
       11. The method of  claim 1 , wherein the casing wear is computed as a casing wall thickness, a casing wall safety margin, or an estimated probability of casing integrity loss. 
     
     
       12. A drilling system that comprises:
 a logging tool that obtains locations of survey points along open portions of a borehole, the locations representing a borehole trajectory of the borehole; 
 a casing string positioned to stabilize at least a portion of the borehole after said locations have been obtained; 
 a drilling rig that drives a drill string to extend the borehole; and 
 a processing unit that, based at least in part on said locations of the survey points, estimates casing wear for the casing string, said estimating including: 
 deriving, using the locations of the survey points, locations of adjusted survey points that represent a casing trajectory along the borehole based at least in part on a bending stiffness of the casing string; 
 computing, as a function of position along the casing string, the casing wear based at least in part on the casing trajectory and one or more of: a bending stiffness of a drill string, a rotational speed of the drill string, a dimensional property of the drill string, a drilling time, and a wear factor. 
 
     
     
       13. The system of  claim 12 , wherein the processing unit comprises casing wear estimation software stored on a non-transitory information storage medium. 
     
     
       14. The system of  claim 13 , wherein the software causes the processing unit to display the casing wear as a log. 
     
     
       15. The system of  claim 12 , wherein the software employs at least one of a stiff string model and a finite element model to account for bending stiffness of the casing string. 
     
     
       16. The system of  claim 15 , wherein the software further employs the stiff string or finite element model to account for bending stiffness of the drill string. 
     
     
       17. The system of  claim 12 , wherein said computing includes determining a side force of the drill string against the casing string using a soft-string model. 
     
     
       18. The system of  claim 12 , wherein said computing includes determining a side force of the drill string against the casing string. 
     
     
       19. The system of  claim 12 , wherein said deriving further accounts for one or more casing string centralizers on the casing string. 
     
     
       20. The system of  claim 12 , wherein said deriving is based at least in part on a pre-casing caliper log. 
     
     
       21. The system of  claim 12 , wherein the casing wear is computed as a casing string wear volume or a casing string wear groove depth. 
     
     
       22. The system of  claim 12 , wherein the casing wear is computed as a casing wall thickness, a casing wall safety margin, or an estimated probability of casing integrity loss.